Why is TurboTax not reducing in come for Excess Business Interest Expense from my K-1. It refers me to an external form, and does not decrement line 1 by line 13k.

Excess Business Interest Expense must be offset by business income and the deductible amount is subject to certain limitations. The actual amount to be used is calculated on Form 8990, which is not supported by TurboTax for Form 1040 tax returns.  

If you want to track your excess business interest, you will need to manually prepare that form and include it with the return that you print and mail. (You can't e-file a return that includes Form 8990.)

Click here for IRS form and instructions for Form 8990.